1. Field of the Invention
The present invention relates to a heterodyne interferometric optical fiber displacement sensor. More particularly, this invention is concerned with a heterodyne interferometric optical fiber displacement sensor capable of measuring a displacement of an object and an absolute distance from the sensor to the object using a laser diode whose oscillation frequency can be modulated linearly by a drive current.
2. Description of the Related Art
Heterodyne interferometric displacement sensors based on frequency modulation have been known in the past. One example is described in Japanese Patent Application Laid-open No. 63-101702. In this sensor, a triangular waveform current is applied to a laser diode in order to modulate the oscillation frequency and light emission intensity of the laser diode. Then, the laser beam is divided into reference light and object light by a beam splitter. The object light irradiates an object's surface. The reflected light, which is light reflected from the object's surface, is then superimposed on the reference light. A time lag occurs between the reflected light and the reference light because of a distance from the sensor to the object's surface. This time lag causes the frequencies of the reflected light and the reference light to differ from each other. Therefore, the reference light and object light cause a heterodyne interference. The beat frequency is associated with the distance from the sensor to the object's surface. Therefore, when the beat frequency is measured, the displacement of an object under test can be obtained.
However, since the foregoing conventional heterodyne interferometric displacement sensor does not employ any optical fibers as light paths of the laser beam, it is sensitive to any environmental disturbance. It is large in size because of incorporation of the beam splitter. Furthermore, to measure the beat frequency correctly, measurement is only carried out when the object is stationary. This hinders the real-time measurement of a moving object.
On the other hand, a photothermal displacement detection optical fiber interferometer that employs an optical fiber as a light path of laser beams has been proposed in Japanese Patent Application Laid-open No. 63-82344. However, in this interferometer, a half mirror and a dichroic mirror are placed between a light source and the optical fiber. Owing to the mirrors, excitation light emitted from an excitation light source for displacing a sample photothermally and detection light emitted from a detection light source for detecting a photothermal displacement using optical interference enter one end of an optical fiber, and interference light that is reflected from an emission surface at the other end of the optical fiber and from a sample surface and returned through the optical fiber enters a photoelectric converter. Therefore, no optical fiber is present between the light source and one end of the optical fiber, or between one end of the optical fiber and the photoelectric converter. Consequently, the interferometer is, like the aforesaid prior art, easily affected by an environment and large in size.